Multiphase DC/DC converters with a configuration in which a plurality of voltage conversion units are connected in parallel to each other are known as DC/DC converters that drive switch elements to step up or down a DC voltage. Examples of this type of multiphase converter include a technique as disclosed in JP Patent No. 4452383, and in this technique, the number and combination of switching circuits to be activated are varied depending on the output current.
Meanwhile, in this type of DC/DC converter, soft start control of gradually increasing a target value for output is performed in the initial stage immediately after the start of output, in order to prevent an inrush current at the start of output. The soft start control employs, for example, a control method of gradually increasing a control target current value with a constant climb gradient toward a final target current value, and performing feedback control such as PID control based on the control target current value, so that the output is increased.
However, if soft start control is performed immediately after the start of output in this way, then a time period will be created in which an inductor current falls below 0, due to a reduction in the amount of output current during the soft start control, and the output current flows reversely in this time period. Because such a reverse flow state will cause a large loss, some sort of countermeasure is required, but no such countermeasure against a reverse flow state is proposed in the technique of JP Patent No. 4452383.
For example, FIG. 4 schematically shows, in a synchronous rectification type multiphase converter that has the same hardware configuration as that of FIG. 1 except for the control unit 3, the current waveform of a single phase during the soft start control performed thereon. In FIG. 4, “Va” indicated by a dotted line schematically shows an average current value. In FIG. 4, the waveform of an inductor current immediately after the start of the soft start control is indicated by a solid line, and when the output current (average current Va) is small as shown in FIG. 4, the inductor current falls below 0 in time periods before and after a rising flank of a high-side FET, and reverse flows occur in these time periods (see areas A1 and A2 of FIG. 4).
In order to prevent such a reverse flow, it is conceivable, as shown in FIG. 5, to switch off the low-side FET in the time periods in which the reverse flow occurs, for example. With this, it is possible to interrupt the conduction of a reverse flow path from an output line to the ground via the low-side FET, and prevent the reverse flow. However, in this method, PWM control using hardware or software is complicated.
As another method, as shown in FIGS. 6(A) to (C), a method is conceivable in which, similar to the configuration shown in FIG. 1, reverse flow preventing FETs (elements similar to switching elements SC1, SC2, SC3, and SC4 of FIG. 1) are arranged for respective phases, and during soft start control for each phase, the reverse flow preventing FET of the phase is kept in the OFF state, and the output current of the phase is rectified by its parasitic diode. However, in this method, the loss is larger than in a case where output is performed while the reverse flow preventing FETs for all the phases are switched on.
The present invention was made in view of the above-described circumstances, and it is an object thereof to realize a configuration that can subject, when operation of a multiphase conversion unit is started, voltage conversion units to control of gradually increasing a target value for output, and can suppress a reverse flow, more easily and further avoiding a loss.